This invention relates to lasers and more particularly to an improved gas discharge laser.
One type of gas discharge laser is the excimer laser. An excimer laser has a lasing gas with a molecule which exists only in the excited state. The atoms comprising the gas, for example, krypton (Kr) and fluorine (F.sub.2) exist as such in the laser prior to excitation and, when excited by an electric discharge, combine to form the molecule KrF which emits photons at its characteristic wavelength 249-nm while dropping from the upper energy state to the lower or ground state. Upon reaching the ground or terminal state, the molecule disassociates into its constituent atoms. Accordingly, saturation at the lower energy state cannot occur in an excimer laser which overcomes power output limitations attributed to this cause. This laser therefore has considerable promise in generating outputs at visible and ultraviolet wavelengths which have utility in such applications as isotope separation, nuclear fusion and photochemistry. Other excimer and excimer-like lasers utilize gas lasing media consisting of argon (Ar) and fluorine (F.sub.2), xenon (X.sub.e) and F.sub.2, Xe and bromine (Br.sub.2), mercury (Hg) and Br.sub.2, Hg and chlorine (Cl.sub.2), and Xe and Cl.sub.2, which form the excimer or excimer-like substances argon fluoride (ArF), xenon fluoride (XeF), xenon bromide (XeBr), mercury bromide (HgBr), mercury chloride (HgCl) and xenon chloride (XeCl), respectively.
In the operation of a high pressure pulsed excimer laser, good electric discharge uniformity is essential to efficient laser performance. In order to provide a uniform high voltage discharge in high pressure gases, the gas must be pre-ionized immediately prior to the application of the main discharge pulse. "Pre-ionization" means that a uniform ion or electron cloud is generated in the gas in the discharge region which acts to uniformly "seed" the main discharge. This uniform ion cloud tends to prevent the formation of arcs or streamers when the main discharge occurs, thereby providing for a more spatially uniform discharge.
In excimer lasers, as in other gas discharge lasers, it is necessary to provide a buffer gas in the gas mixture in order initially to support the discharge since, by definition, the excimer molecule exists only in the excited state. In prior practice, the buffer gas used has been helium (He) because it is chemically inert, inexpensive, readily available, has a high ionization potential and forms stable low pressure discharges. It is light and has a high specific heat so that it is used to rapidly remove excess discharge heat (as in the CO.sub.2 laser discharge for instance). In conventional pulsed-discharge high-pressure excimer lasers, discharge uniformity is enhanced by irradiating the discharge region with ultraviolet (u.v.) radiation generated by an array of sparks. The u.v. radiation from these spark discharges ionizes the gas in the main discharge volume providing the required pre-ionization. The operation of the laser requires that the spark discharge occur some time prior to the main discharge; that is, energization of the spark array is required to occur some time (typically less than 1 microsecond) prior to application of the main voltage pulse to the primary electrodes in the system. Some delay between the spark array discharge and main voltage pulse is required to allow time for the u.v. intensity from the sparks to reach maximum value.
While the spark array technique provides sufficient pre-ionization to permit effective use of He as a buffer gas, it has several disadvantages. It requires separate energy storage cicuitry which lowers the overall laser efficiency. It requires separate timing circuitry to delay the main discharge pulse some hundreds of nanoseconds after the initiation of the spark discharges. Further, the spark array electrodes tend to erode away as they are discharged, the eroded material and the freshly exposed electrode material thereby contributing to gas contamination and reduction in laser efficiency.